1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming system, and a transfer method.
2. Description of the Related Art
An image forming apparatus of an electrophotographic system forms an electrostatic latent image on a uniformly-charged image carrier, develops the formed electrostatic latent image with toner to form a toner image, and transfers and fixes the formed toner image onto a recording sheet to thereby form an image on the recording sheet.
A recording sheet usually has irregularities and toner is less easily transferred to recesses than to projections. Therefore, when an image is formed on a recording sheet having large irregularities, in some cases, toner is not transferred to recesses and density unevenness, such as white voids, occurs on an image.
Therefore, for example, Japanese Patent Application Laid-open No. 2007-304492 discloses a technology for specifying, from a difference between current values of electric currents flowing through two metal roller pairs, irregularities of a recording sheet that passes through the two metal roller pairs and adjusting a toner adhesion amount to be an adhesion amount suitable for the specified irregularities.
However, in the conventional technology described above, while the amount of toner deposited on a recording medium can be set to an amount suitable for the irregularities, a toner transfer ratio to the recording medium is not improved. Therefore, density unevenness of an image cannot be reduced.
As a method for reducing the density unevenness of an image even when the image is formed on a recording medium having irregularities, there is a method for transferring an image to a recording medium by selectively applying a direct-current voltage or a voltage based on at least an alternating-current voltage to a transfer unit depending on the degree of irregularities of the recording medium.
However, in this method, the rise time of the voltage based on at least the alternating-current voltage tends to be longer than the rise time of the direct-current voltage, and this sometimes causes density unevenness or density reduction of an image.
Therefore, there is a need for an image forming apparatus, an image forming system, and a transfer method capable of reducing density unevenness or density reduction of an image even when a voltage used for transferring an image is changed depending on a recording medium.